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From: Warner Losh <imp@bsdimp.com>
To: Noel Chiappa <jnc@mercury.lcs.mit.edu>
Cc: TUHS main list <tuhs@minnie.tuhs.org>
Subject: Re: [TUHS] Memory on Lion's v6
Date: Tue, 1 Mar 2022 19:15:43 -0700	[thread overview]
Message-ID: <CANCZdfrXVuPr4waooiQUG7na98iBJ++8tML2rKX94T-=y6_C7g@mail.gmail.com> (raw)
In-Reply-To: <20220302011959.0B3D818C087@mercury.lcs.mit.edu>

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On Tue, Mar 1, 2022, 6:23 PM Noel Chiappa <jnc@mercury.lcs.mit.edu> wrote:

>     > From: Andrew Hume
>
>     > the actual configuration of Lions; PDP 11/40 was
>     >        128 Kbytes of core memory
>     > ...
>     > but note that because ... of addressing weirdness (the top 8KB were
>     > memory-mapped to I/O registers), Lions' PDP actually had 112KB of
> main
>     > memory
>
> I think that '112KB' must be an error; the 8KB for the 'I/O page' (as DEC
> eventually named ir, long after the rest of the world had started using the
> term :-) were deducted from the _UNIBUS_ address space, meaning a UNIBUS
> -11
> (the 'pure' UNIBUS -11's, i.e. other than the -11/70, -11/44, etc) could
> have
> a maximum of 248KB of main memory (which is on the UNIBUS).
>
> A pure UNIBUS -11 with 128KB of main memory (like Lions') has... 128KB of
> main memory. The 'small memory management model' -11's (like the /40, /60,
> /23, etc) can use at most 64KB of that _at any moment in time_ for user
> processes (i.e. directly accessible by the CPU, in 'user' mode).
>
> (The kernel on such machines is basically retricted to 56KB at any moment
> in
> time, since one 'segment/page' - the terminology changed over time - has
> to be
> dedicated to the I/O page: the memory management control registers are in
> that, so once the CPU can no longer 'see' them, it's stuck. Long,
> potentially
> interesting digression about, and ways to semi-work around that, elided,
> unless people want to hear it.)
>
>
>     > From: Noel Chiappa
>
>     > The -11/40 (as it was at first) that I had at LCS had, to start with,
>     > I'm pretty sure, 3 MM11-L units .. - i.e. 48KB. I know this sounds
>     > incredible, and I'm having a hard time believing it myself, wondering
>     > if my memory is failing with age
>
> It is:
>
>   # size /lib/c0
>   13440+2728+10390=26558 (63676)
>
> ('c1' takes 14848+6950+2088=23886, FWIW.) So 'my' -11/40 must have had more
> than 48KB.
>
> MINI-UNIX provides, on an -11/05 type machine with the maximum of 56KB of
> addressable main memory (if you plugged in 64KB worth, the /05 CPU couldn't
> 'see' the top 8KB of that), up to 32KB for a user process. So that will
> just hold the stock V6 C compiler.
>

You made a comment that MINI-UNIX wasn't available outside of Bell... I
meant to say that the AUUG newsletters talk about it. It features a letter
asking for users of it to share patches. There was also an article about
how to get it, though it was an offer to spin a tape for a photocopy of you
Western Electric license...

Warner

I'm not now sure how much memory my -11 _did_ have initially, but it's not
> important.
>
>         Noel
>

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  reply	other threads:[~2022-03-02  2:19 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-02  1:19 Noel Chiappa
2022-03-02  2:15 ` Warner Losh [this message]
  -- strict thread matches above, loose matches on Subject: below --
2022-03-02  2:26 Noel Chiappa
2022-03-01  0:44 Noel Chiappa
2022-02-28  5:48 Will Senn
2022-02-28  6:09 ` Warner Losh
2022-02-28 15:48 ` Clem Cole
2022-02-28 23:27   ` Andrew Hume
2022-03-01 15:31     ` Andrew Hume
2022-03-01  1:25 ` Bakul Shah

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